Since the introduction of laminate-type organic electroluminescent (hereinafter this may be abbreviated as “EL”) elements into the market by Kodak using a vapor deposition method, organic EL displays have become actively developed and are now being put into practical use.
The laminate-type organic electroluminescent element of the type comprises plural organic layers (luminescent layer, hole injection layer, hole transport layer, electron transport layer, etc.) as laminated between the anode and the cathode therein. The laminate-type organic electroluminescent element of the type is self-luminous and therefore has a broad viewing angle and high visibility, and in addition, it is thin and therefore attracts special attention from the viewpoint of space saving.
Another characteristic feature of the organic electroluminescent element is the planar luminescent performance thereof different from that other luminescent diodes and cold-cathode tubes that have heretofore been put into practical use in the art. As applications that make use of the characteristic feature, there are mentioned backlights, lighting devices, full-color display devices using a color filter, etc.
Organic electroluminescent elements enable various light emissions depending on the combination of the constituent materials. For example, in case where luminescent materials for emission of red, green and blue are used, or in case where luminescent materials for emission of blue and yellow complementary to each other are used, white emission is possible; and in case where luminescent materials for emission of red and green, yellow emission is possible.
These days, in particular, applications of planar light sources such as lighting devices, backlights and others that utilize such color mixing systems are expected. For such lighting applications, preferred is use of a white light source and an electric lamp color light source.
For producing a white emitting element, for example, there is mentioned a method of adding plural kinds of luminescent materials for emission of red, green and blue are added to one layer, as described above. In case where an organic electroluminescent element is produced according to a vacuum vapor deposition process that employs the method, at least red, green and blue luminescent materials and a charge transporting material, totaling at least four different kinds of materials must be vapor-deposited all at a time. In a vacuum vapor deposition method, it is difficult to control vapor deposition of 3 or more different kinds of materials, and the method therefore has a problem in that the repetitive reproducibility is poor (PTL 1).
On the other hand, in case where an organic electroluminescent element is produced according to a wet film formation method, in general, the luminescent material is dissolved in a solvent (PTL 2). In this case, when organic metal complexes each having a ligand having the same basic skeleton are used as the red, green and blue luminescent materials, precipitates may form in the solution, and even when precipitates do not form in the solution, there may occur aggregation during coating or drying, and therefore there may be a possibility that the luminescent material of the type could not form uniform films along with the other luminescent materials and the charge-transporting material. Consequently, there may be a possibility of providing other problems of color shift and poor repetitive reproducibility.
Further, in a case where a luminescent layer is formed according to a wet film formation method, a polymer material may be used (PTL 3). However, the molecular weight of a polymer material is difficult to control and, as compared with a low-molecular material, it is impossible to sufficiently purify a polymer material. Accordingly, in a case where a phosphorus material is used as a luminescent material, there occurs a problem of reduction in the luminescent efficiency. Further, in a case where a luminescent layer is formed using a low-molecular material alone and where one compound alone is used as the charge-transporting material, it is considered that the crystallinity of the charge-transporting material is high and therefore could not have a sufficient amorphous structure. As a result, there may occur some problems of driving voltage increase and light emission efficiency reduction (PTL 4).